1. Field of the Invention
The invention relates to thermo-optic switches, and more particularly to methods and structures to achieve fast switching rise times in thermo-optic switches, primarily but not exclusively for display applications.
2. Description of Related Art
Referring to FIG. 1A, guided wave devices typically consist of an optical path defined by at least a core 115 and a cladding 110/120 that confines the optical path in two dimensions. The core layer 115 is adjacent to one or more cladding materials 110/120 that have a lower refractive index than the core. In the illustration shown, the substrate itself forms a lower cladding 120 for confinement normal to the plane of the surface, while either air or a material deposited on the core forms an upper cladding 110 to complete the confinement normal to the plane. In some glassy or crystalline materials, the core 115 of the waveguide can be formed by diffusion of an ion into a substrate, raising the index of refraction. In this case, both the core layer 115 and lower cladding 120 are part of the substrate. In other materials such as polymers, the core and cladding are typically deposited in layers, with a core layer 115 surrounded by lower 120 and upper 110 cladding layers to provide confinement for the waveguide normal to the plane. Confinement in the second dimension, the plane of the substrate, can be provided by either a difference in thickness or refractive index of a portion 135 of the core layer 115. Optical waveguides may have many forms, such as channel waveguides described above, planar waveguides and optical fiber waveguides for example.
Thermo-optic (xe2x80x9cTOxe2x80x9d) switches may be formed using any waveguide forms including but not limited to those mentioned above. TO switches operate on the principle of a thermally-induced change in index of refraction of the optical path at a switch location. Thermo-optic devices are useful for many applications because of polarization insensitivity, the availability of low-loss thermo-optically active materials, and the absence of charging affects associated with EO devices.
As illustrated in FIG. 1A, a conventional TO device 100 typically includes a resistive heater 105 which, by injecting thermal energy through a top cladding layer 110 into the core 115, increases the temperature in the core and changes its refractive index, forming an index-modified region 125. The index-modified region acts as a switch, causing the light propagating along 130 to be diverted from the waveguide. The resistive heater 105 is shown symbolically in the figure and the switch could be any optical switch known in the art including, but not limited to, Mach-Zehnder interferometers, directional couplers, two-mode interferometers, and total internal reflection (TIR) devices. The switch is activated by applying a control signal, such as a voltage or current, to the resistive heater 105.
The prior art discloses two different regimes of operation for thermo-optic switches: one regime in which the electrical power is applied continuously to the heater so that the deflection efficiency of the switch approaches a constant steady-state value during application of the electrical power (sometimes referred to herein as xe2x80x9cregime Ixe2x80x9d or a xe2x80x9csteady-state regimexe2x80x9d), and a second regime in which electrical power is applied in a drive pulse that ends before a steady-state deflection efficiency is reached (sometimes referred to herein as xe2x80x9cregime IIxe2x80x9d or an xe2x80x9coverdrive regimexe2x80x9d), such that the response time of the device is approximately equal to the drive pulse width.
For the purpose of clarity, we specifically define a device to be operating in the steady state regime when the change in deflection efficiency of the device exceeds 90% of the maximum deflection efficiency change that occurs as a result of a specific control pulse for at least one-half the length of the control pulse. Contrarily, a device is specifically operating in the overdriving regime when the change in deflection efficiency of the device exceeds 90% of the maximum deflection efficiency change that occurs as a result of a specific control pulse for less than one-half the length of the control pulse, and is not otherwise operating in a third regime, the xe2x80x9cnear-impulse response regime,xe2x80x9d which is defined elsewhere in this document.
FIG. 1B illustrates the amplitude of the control signal over time for a switch operated in the steady-state regime. FIG. 1C illustrates the resulting deflection efficiency response of the switch. As shown in FIG. 1B, in steady-state operation of the switch, the control signal, for example a voltage or current, is applied to the resistive heater 105 of the TO device 100, causing the heater to inject thermal energy into to optical path, thereby increasing the temperature of the material in the optical path 130 near the resistive heater 105, forming an index-modified region 125. During steady-state excitation shown in FIG. 1C, the temperature of the core 115, as well as the low power deflection efficiency of the device, asymptotically approaches a steady-state maximum value. The deflection efficiency of a device is defined herein as the percentage of optical energy that was originally in the optical path 130 that is diverted from the optical path 130 as a result of switch activation. With reference to deflection efficiency, low power implies non-saturation of the deflection efficiency response; i.e., the index of refraction does not exceed the critical index of the device during the pulse so that the shape of the deflection efficiency response is similar to that of the index response. Once the device reaches steady-state, the deflection efficiency and thermally-induced refractive index do not change until the control signal changes. Typical switch rise and fall times reported for switches operated in the steady-state regime in a polymer material system are on the order of 0.5-9 ms.
In the second regime (II) of operation for thermo-optic devices disclosed in the prior art has been referred to as (xe2x80x9coverdrivingxe2x80x9d), an electrical energy pulse applied to the optical heater ends before a steady state optical response is reached. FIG. 2A illustrates a control signal operating a TO switch in the overdrive regime, and FIG. 2B illustrates the deflection efficiency response. Referring to FIG. 2B, the deflection efficiency of the device operated in this regime continues to increase during the entire time that the electrical drive pulse shown in FIG. 2A is applied. The deflection efficiency never saturates so that the device never reaches a steady state; thus, the response time from the start of the drive pulse to the peak deflection efficiency is approximately equal to the pulse width. The thermo-optic response to heat pulses in this regime has been analyzed by several authors, and Nishihara et al disclose an approximate expression to calculate the transient surface temperature for pulsed operation in Optical Integrated Circuits, New York: McGraw-Hill, 1989. Typical switch response times reported for thermo-optic switches operated in the overdrive regime are on the order of 75-200 xcexcs in polymer material systems.
Some applications, such as fiber-optic routers for communications signals and optical displays, require faster rise times than can be obtained with the prior art operating in the first two regimes. Commonly assigned Bischel et al. U.S. Pat. No. 5,544,268 for xe2x80x9cDisplay Panel with Electrically-Controlled Waveguide-Routingxe2x80x9d, describes two-dimensional addressable electro-optical switch arrays used to provide flat panel video displays. In these devices, fast switch rise times are required in order to sequence through an entire row of switches at a rate appropriate for display applications. By incorporating the invention described herein, faster responses can be achieved compared to methods discussed in literature.
It is an object of this invention to provide a method for minimizing the rise time of a thermo-optic (TO) device while simultaneously maximizing the device lifetime. Roughly described, this object is achieved in part by operating the device in a third regime (III) of operation not before disclosed. In this regime, sometimes referred to herein as the near-impulse response regime, the drive pulse width is reduced to a value that is less than two times the diffusion time of the switch so that the drive pulse has a width that is less than the rise time of the steady state optical response and less than the rise time of the deflection efficiency response of the applied over-drive pulse. By comparison, the width of the drive pulse in the second regime is approximately equal to the response time of the deflection efficiency to the applied drive pulse. Various other techniques can also be used to help reduce the switch rise time.
In another embodiment of this invention, a sustaining pulse is combined with an initial pulse in regimes I or II, in order to extend the ON time of the device while maintaining a fast rise time and maximizing the device lifetime.